Most entertainment lighting applications require a means to selectively vary the apparent color of the light beam produced by the fixtures employed.
For this purpose, virtually every lighting fixture designed for the application provides some means to support at least a planar filter material in its beam. Beam color may be changed by changing the filter material.
This filter material or "gel" generally consists of a flexible gelatin, cellulose acetate, mylar, or polyester base with a colored pigment disposed through the base or applied to it. Five major lines of flexible filter material are available, providing the lighting designer with a choice of more than 200 distinct colors.
While the cost of the filter material itself is modest, the requirement that a change in beam color requires changing filter material has always had disadvantages in those applications that require the color of the beam illuminating a given subject change during use. Without a method of remotely changing the apparent color of the beam produced by a fixture during a performance, lighting a given area of the stage in five colors generally requires the use of five fixtures, each provided with a different filter material but otherwise identical--at a very substantial increase in both direct and indirect cost in fixtures, cabling, dimming, support, and control equipment, as well as in the manpower required to install, adjust, and service this amount of equipment.
It has long been apparent that if a fixture's color could be changed by remote control during the performance, that the number of fixtures required and as such the direct and indirect costs of the total lighting system could be dramatically reduced. As a result, methods of changing the color of a light source from a remote location found use with candles in the 1770s; with electric lights in the 1880s; and electrically actuated changers similar to those disclosed in U.S. Pat. Nos. 2,129,641 and 2,192,520 were in significant use in American theater by the 1930s.
The ultimate extension of the theory that a fixture with remotely variable beam characteristics could produce a significant reduction in the size of the lighting system required for a production leads us to a fixture, as disclosed in U.S. Pat. No. 3,845,351, whose every beam variable is under the control of an electronic memory system. A practical color changer is essential to its success in most applications.
Methods for varying the color of an entertainment lighting fixture's beam in the past have been largely restricted to the use of "gel changers"; mechanisms providing one filter of appropriate size for each beam color desired, and that mechanically displace these filters in and out of the beam to change color as required. Various designs for such "filter changers" have been disclosed, but in operation they may be divided into two classes which may be described as "serial" and "parallel".
In "serial" changers, the filters are attached to a common mechanical support that is displaced relative to the beam in order to insert the desired filter. One of the class is the color wheel as disclosed in U.S. Pat. Nos. 1,820,899 and 2,214,728; a disc supporting a number of filters spaced at a common radii around the hub, the disc rotated to change color. Recently, "roller" changers similar in principle to the unit disclosed in U.S. Pat. No. 3,099,397, which scroll gel strips (produced by fastening squares of flexible filter material to each other) between two reels have come into significant use. Both types of "serial" changers share inherent disadvantages; the number of colors produceable is limited to the number of filter positions as no subtractive combination of two filters is possible. Changing between filters/colors also frequently requires passing through other, unwanted filters/colors occupying intermediate positions on the strip or wheel.
In "parallel" changers, the second major class, each filter is provided with a separately actuated support which displaces it in and out of the beam. The most common type is the "semaphore" changer as disclosed in U.S. Pat. Nos. 2,129,641 and 2,192,520. Such changers allow subtractive combinations of more than one filter/color; "white" light without sacrificing a filter position; and changing between any two filters without passing through others (although semaphore changers are not without their own undesirable intermediate effects during slow color-to-color changes.)
Some hybrids of the "serial" and "parallel" approaches have also been used including parallel changers with two colors per filter support and fixtures with two or more coaxially mounted color wheels or scrollers, but such hybrids cannot mitigate the disadvantages of a type without also mitigating its advantages.
Both "serial" and "parallel" changers have had several important disadvantages:
One is the undesirable intermediate colors such changers can produce during color-to-color transitions. As a consequence, such so-called "color changers" cannot be used to truly change the color of the fixture beam during use. Either the undesirable intermediate colors will be seen--which is unacceptable by modern professional standards. Or the luminaire will have to be turned off during the transition--which is generally impractical. Or two luminaires with two color changers must be provided so that alternating color changers may "preset" to the desired filter and the transition be accomplished by dimming between the two fixtures--the added cost and complexity of the arrangement mitigating most if not all of the benefits of color changer use.
Another disadvantage of such filter changing systems is that the number of colors produceable is limited to the number of filters provided plus the number of subtractive combinations of multiple filters allowed by the mechanical design of the changer itself.
Therefore it has long been desirable to produce a simple and economical system that allows the synthesis of substantially any desired color using a limited number of color filters and that allows color-to-color transitions without undesirable intermediate effects.
Color theory states that either additive or subtractive primaries can be used to synthesize sensations equivalent to those produced by discrete filters.
At least one model of remotely-adjustable lighting fixture has employed a three-color subtractive color changer generally equivalent to that disclosed in U.S. Pat. No. 3,883,243. Such changers provide filters for each of the subtractive primaries, each such filter mechanically displaced relative to the beam to vary the proportion of energy in the beam in the bandpass region of that filter. Such subtractive techniques have apparent limitations in producing saturated color sensations.
The additive primary system has long been used in many fields to produce a range of color sensations, in the theater, notably for borderlights and cyclorama lighting.
Applied to the design of a fixture, one approach has been to provide a single fixture with three light sources, each separately filtered and controlled in intensity, the output of all three sources preferably combined prior to any beam forming elements in the fixture's optical path; an approach disclosed in U.S. Pat. No. 4,071,809. The disadvantage of this approach is the requirement for three light sources with their associated direct costs and power requirements to produce the same beam intensity as a conventional fixture with a single source.
A more power efficient approach is to derive all three additive primaries from a single light source by passing the beam through a series of filters, dividing it into three beams; mechanically douse each one; and then recombine them prior to the beam forming elements in the fixture's optical system, an approach disclosed in U.S. Pat. No. 3,818,216. The disadvantages of this approach include the requirement for mechanical dowsing (electronic dimming of the light source being impractical) and the size, complexity, cost, and light losses of the optical system required.
It has therefore long been desirable to produce a three-color additive mixing fixture suitable for entertainment lighting use that requires only a single source and a single optical path. A color mixing light bulb theoretically suitable for this purpose is disclosed in U.S. Pat. No. 3,225,243, but leaving aside the cost of producing such a bulb and its socket, the difficulty of imaging all nine filaments in a collector of reasonable size will be apparent.
It is the object of the present invention to provide an improved additive color mixing fixture of greater simplicity and efficiency, practical for use with existing light sources and fixture designs.